1. Field of the Invention
The present invention relates to a radiation image reading apparatus for reading radiation images from an accelerated phosphorescence fluorescent material object on which radiation images of a subject are accumulated and stored and, more particularly, a radiation image reading apparatus capable of obtaining image signals which carry radiation images with high contrast resolution.
2. Description of the Related Art
Radiation images such as X-ray images have been often used for medical diagnoses. For example, in a case of X-ray images, an X-ray which has transmitted through a subject is irradiated onto a fluorescent material layer (fluorescent screen), then converted to a visible light, and this visible light is irradiated onto a silver halide film to form a latent image and the X-ray image is obtained by developing this silver halide film. The X-ray images thus obtained are used in medical diagnoses. Recently, a system for obtaining high quality reproduced images has been employed by which the X-ray image formed on a silver halide film as described above is read photoelectrically by a so-called film digitizer to obtain image signals and these image signals are image-processed to improve various image characteristics which determine those image qualities such as definition, dynamic range, graininess and others.
Instead of the above described system using silver halide films, a system which uses phosphorescence fluorescent material (accelerated phosphorescence material) has begun to be used. This system using accelerated phosphorescence fluorescent material is intended to accumulate and store an X-ray image on an accelerated phosphorescence fluorescent panel (including a sheet), which is made up by forming accelerated phosphorescence fluorescent material in a shape of panel (or sheet) by irradiating the X-ray which has transmitted through the subject, photoelectrically read this X-ray image to obtain image signals and further obtain a reproduced image after these image signals have been imageprocessed. The basic method of this system is disclosed in the U.S. Pat. No. 3,859,527. For the purpose of this specification, the "accelerated phosphorescence fluorescent material" is a fluorescent material which accumulates therein part of energy of a radiant ray for a while or a long period of time when an radiant ray such as, for example, the X ray, .alpha. ray, .beta. ray or .gamma. ray is irradiated and emits the accumulated energy as an accelerated phosphorescence fluorescent light when an excitation beam such as, for example, infrared radiation, visible light or ultraviolet radiation is irradiated during such accumulation. In this case, a type of radiant ray the energy of which is prone to be accumulated, a wavelength of an excitation beam by which an accelerated phosphorescence fluorescent light is easily emitted and a wavelength of an accelerated. phosphorescence fluorescent light to be emitted differ depending on the type of a fluorescent material to be used.
According to a system using this accelerated phosphorescence fluorescent material, it has been recognized that an energy of radiant ray irradiated onto this accelerated phosphorescence fluorescent material is proportional to a light quantity of the accelerated phosphorescence fluorescent light emitted by irradiation of the excitation beam in a wide range of energy and this ratio of proportion can be changed in accordance with the light quantity of the excitation beam and therefore a radiation image which will not be affected by the variations of the dose of radiant ray can be obtained. In a case of the system for obtaining X-ray images of a human body, the exposure dose to a human body can be reduced in the X-ray radiation photography.
FIG. 17 is an approximate configuration of the conventional radiation image reading apparatus for use in the system employing the accelerated phosphorescence fluorescent material.
In a camera system not shown, a subject is positioned in front of the photography stand, the X-ray generated from the X-ray generator is irradiated to the subject, the X ray which has transmitted through this subject is irradiated onto an accelerated phosphorescence fluorescent panel set on the photography stand and an X-ray image is accumulated and stored on this accelerated phosphorescence fluorescent panel.
After the photography has been carried out as described above, the accelerated phosphorescence fluorescent panel 3.sub.-- 1 is taken out from the photography stand and set on a precision slide 3.sub.-- 7 of the radiation image reading apparatus as shown in FIG. 17. The accelerated phosphorescence fluorescent panel 3.sub.-- 1 set on the precision slide 3.sub.-- 7 is transferred (for sub-scanning), as is being set, in the Y direction shown with an arrowhead by transfer means not shown.
During such transfer (sub-scanning), a laser beam 3.sub.-- 2 as an excitation beam of a wavelength of, for example, 780 nm, emitted from a semiconductor laser 3.sub.-- 4 is repeatedly reflected and deflected by a scanner 3.sub.-- 5 such as, for example, a galvanometer mirror or a rotary polygon mirror and irradiated onto the accelerated phosphorescence fluorescent panel 3.sub.-- 1 after passing through a beam shape correcting optical system 3.sub.-- 6 such as a f.theta. lens whereby this accelerated phosphorescence fluorescent panel 3.sub.-- 1 is repeatedly scanned (main scanning) in the X direction shown with an arrowhead by the laser beam 3.sub.-- 2. From respective scanning points are emitted the accelerated phosphorescence fluorescent light which carries the X-ray image which is accumulated and stored on the accelerated phosphorescence fluorescent panel 3.sub.-- 1. This accelerated phosphorescence fluorescent light is condensed by a condenser 3.sub.-- 8, led to a photomultiplier tube 3.sub.-- 9 through an optical filter (not shown) which cuts off the excitation beam and simultaneously admits the accelerated phosphorescence fluorescent light, and converted to electric signals.
FIGS. 18 and 19 are respectively an approximate perspective view of a light guide passage (condensing member) to be employed in the conventional radiation image reading apparatus. This light guide passage (condensing member) is used for guiding the accelerated phosphorescence fluorescent light which is linearly generated along the main scanning line to a single photomultiplier tube having a circular photoelectric surface. FIG. 18 shows a bundle type light guide passage made up by binding a number of optical fibers and FIG. 19 shows a light guiding sheet which is formed in the shape of a dustpan by deforming an acryl sheet, which is disclosed in the patent application Disclosure No. 87970-1980.
Electric signals obtained from the photomultiplier tube 3.sub.-- 9 shown in FIG. 17 are amplified by an initial stage amplifier 3.sub.-- 10 to an optimal signal level to an A/D converter 3.sub.-- 11, then converted to digital image signals by the A/D converter 3.sub.-- 11. These digitized image signals are stored in an image memory 3.sub.-- 12. After this, these image signals are converted to the display brightness signals and displayed on a CRT unit not shown and outputted to a film as a hard copy.
For example, in the case of X-ray photography of the thoracic region of a human body, a transmission ratio of the X ray through the lungfield region and parts near the skin is large and that through the mediastinal region including the backbone and the heart is small. Therefore, there has been a problem that, if the photography is carried out under the X-ray irradiating condition that the lungfield region is photographed with an optimal density, the photographic density of the mediastinal region is insufficient and, if the photography is carried out so that the mediastinal region is photographed with an optimal density, the photographic density of the lungfield region is excessive.
To solve this problem, the conventional system using silver halide films has used films each having a small slope of the gamma curve (X-ray dose to photographic density curve) in photography so that the lungfield region and the mediastinal region could be displayed with an optimal density. In this case, there has been another problem that the contrast resolution would deteriorate.
As another conventional method for solving the problem, a photographic technique for varying the intensity of the X ray in the X-ray irradiation field thereof has been invented to irradiate a reduced dose of X ray to a region of a human body such as the lungfield region where an X-ray transmission ratio is large and, on the contrary, an increased dose of X ray, to the mediastinal region. Specifically, X-ray photography in this case is conducted by setting an additional filter, which is made of aluminium for damping the dose of X ray and formed to be thick at a part corresponding to the lungfield region and to be thin at a part corresponding to the mediastinal region, between the X-ray tube and the subject. This method has encountered a problem that the shape of this additional filter need be changed in accordance with the size (physical feature) of the subject and therefore the work is complicated.
On the other hand, in a case of a system using accelerated phosphorescence fluorescent material, there has been a problem that the density resolution would deteriorate in A/D conversion of signals which carry images in a wide range of density, although the accelerated phosphorescence fluorescent material itself yields a wide latitude, and the range of brightness which can be displayed on the CRT screen is small in reproduction display of images on the CRT display unit and therefore the contrast resolution would deteriorate in simultaneous display of a physical region where the dose of transmitting X ray is extremely large and a physical region where the dose of X ray is small on the same display system.
The above problem in the system using the accelerated phosphorescence fluorescent material is further described in detail below.
FIGS. 20, 21(A) and (B), 22, and 23 respectively show a radiation image (FIG. 20) carried by image signals obtained from reading by the radiation image reading apparatus shown in FIG. 17, for example, in the radiation photography of the thoracic region in the system using accelerated phosphorescence fluorescent material, a waveform of image signal (FIGS. 21(A) and (B)) along scanning lines 1 and 2 shown in FIG. 20, a histogram Of image signal values (X-ray intensity) (FIG. 22), and a display gradation curve (FIG. 23).
A curve of variations of an image signal value obtained along scanning lines 1 and 2 with respect to a thoracic image as shown in FIG. 20 indicates that the image signal value largely varies at a region of the subject, for example, as shown in FIG. 21. A histogram obtained with respect to the image as a whole as shown in FIG. 20 indicates that the histogram distributes in a wide range of radiation intensity, for example, as shown in FIG. 22. If the image is displayed using a display gradation curve A shown in FIG. 23 when a histogram showing such wide range of distribution is obtained, an image including both the lungfield region and the mediastinal region can be displayed. However, a problem of unsatisfactory contrast resolution will occur since the gradient of the gradation curve A is small. In the case of the gradation curve B, the image of the lungfield region is displayed with satisfactory contrast resolution but the mediastinal region cannot be displayed. In the case of the gradation curve C, the mediastinal region can be displayed with satisfactory contrast resolution but the lungfield region cannot be displayed.